In the particular case of helicopters, for example, there is a need to ensure force-limiting action in the system that is intended to absorb energy when the helicopter falls at high speed due to a failure of its support system. When the landing gear of a helicopter is of the rocking am type, the bottom end of the shock absorber is hinged to the rocking am and its top end is hinged to the structure of the helicopter, with the shock absorber being disposed substantially vertically so that reaction from the ground acts, via the associated wheel or wheels, tending to pivot the rocking am and to thrust the rod of the shock absorber into the body of said shock absorber. As a result, in the event of a crash landing, since the vertical impact speed is much greater than in normal operation, it becomes necessary to provide force limitation which is capable of coming into play as quickly as possible.
Numerous shock absorbers are already in existence, and some of them include force-limiting means that act in the event of excessive pressure.
In general, the shock absorbers used are of the type comprising a main body in which a rod-piston slides, associated firstly with a diaphragm hydraulic shock absorbing means that acts under normal conditions of use, and secondly with an energy absorbing means having two chambers containing gas under pressure, one of which is at low pressure and the other of which is at high pressure, said means being operative during a crash landing.
The force-limiting means that may optionally be provided are either included within a component on the shock absorber (e.g. a separator piston delimiting the volume of the high pressure chamber), or else they are constituted by a pressure release valve specially designed to act in the event of excess pressure due to the rod of the shock absorber being pushed into the body of the shock absorber at high speed.
An example of the first-mentioned type comprises a shock absorber having a main body at its bottom end and a rod-piston at its top end, the main body widening out downwards to form a high pressure chamber that is delimited by a separator piston which is in turn surmounted by a volume of oil. The rod-piston is hollow so as to delimit a low pressure top chamber adjacent to a volume of oil which is in communication with the oil contained in the chamber of the main body via a throttling device (an orifice through a partition) provided at the bottom end of the rod.
Under normal operating conditions, the vertical reaction from the ground causes the red to be pushed into the main body, thereby compressing the volume of gas contained in the low pressure chamber, with this retraction motion being braked by the throttling device. In contrast, in a crash landing, since the impact speed is much higher, the pressure generated by the throttling device is communicated via the oil from the cylinder to the separator piston, and since the pressure then applied to the separator piston exceeds the inflation pressure of the high pressure chamber, said piston moves downwards, compressing said chamber.
Forces are thus limited "naturally" by the separator piston.
Such a system is in use at present and provides satisfactory operation. However, the structure of the shock absorber lends itself poorly to long retraction strokes. In addition, in the event of a crash landing, oil flows both upwards (passing through the throttling orifice to penetrate into the hollow rod) and downwards (passing into the enlarged portion of the main body to move the separator piston downwards), and as a result hydrodynamic disturbances can be generated under certain conditions of impact.
An example of the second above-mentioned type of shock absorber comprises a shock absorber having a main body at the bottom and a rod-piston at the top, the main body including a high pressure chamber at its bottom end, which chamber is adjacent to a low pressure chamber, being separated therefrom by a flexible membrane that may bear against a perforated rigid support secured to said body, the shock absorber further including a throttling device that brakes displacement of the rod within the main body by throttling the fluid contained in said body and adjacent to the low pressure chamber. The rod-piston is hollow and receives a separator piston delimiting two hydraulic chambers.
One such shock absorber is described in the document FR-A-2 608 242.
The throttling device then provided (which is associated with a cylindro-conical tube that absorbs energy by plastic deformation) includes firstly a plurality of small through orifices organized in a circle and that act when the rod is pushed in at normal speeds, and secondly a central pressure relief valve of larger section than the above-mentioned orifices, said valve being kept in its closed position by conical resilient washers stacked on the rod of said valve, but acting to obtain a larger through passage in the event of the rod being pushed suddenly at high speed into the body of the shock absorber, thereby avoiding excess pressures that might cause damage.
Such a system nevertheless remains relatively complex, requiring several parts that must be adjusted accurately.
In addition, the throttling device suffers from drawbacks or limitations that are inherent to the disposition of the components from which it is made: the assembly comprising the valve rod and the conical resilient washers has relatively high inertia and exhibits friction that it is difficult to minimize, which means that very short response times are not possible, not to mention possible risks of the valve rod jamming. In addition, the flow section through the central opening remains limited (the diameter of the valve rod cannot be reduced too far without weakening it, and the central opening cannot be of very great diameter without the risk of excessively reducing the mechanical strength of the corresponding fixed component).
An object of the invention is to design force-limiting means that provide better performance than the known means outlined above, and capable of responding with a very short response time in the event of high impact speed (as occurs in a crash landing).
Another object of the invention is to design force-limiting means that make it possible simultaneously to provide considerable force limiting and accurate control of the associated headloss so as to control the force which is transmitted.
Another object of the invention is to provide a force-limiting device whose structure is simple and whose manufacturing cost is reasonable, while still being suitable for integration in shock absorbers of conventional design.